There has been a long controversy concerning whether the amygdala's response to emotional stimuli is automatic or dependent on attentional load. Using magnoencephalography and an advanced beamformer source localization technique, we found that amygdala automaticity was a function of time: while early amygdala responding to emotional stimuli (40-140 ms) was unaffected by attentional load, later amygdala response (280-410 ms), subsequent to frontoparietal cortex activity, was modulated by attentional load.
"A recent study found that neurons in the macaque pulvinar can respond selectively to snakes in 55 ms (Van Le et al., 2013), which is likely too short for a cortical route. It has also been found that the amygdala can be activated with low latencies from a fear-relevant stimulus in about 40–120 ms (Luo et al., 2010), perhaps along the low road. While dual pathways were initially observed in rats, there is functional evidence this applies to primates and specifically humans (Rudrauf et al., 2008; Garrido et al., 2012; Garvert et al., 2014) as well. "
[Show abstract][Hide abstract] ABSTRACT: A hypothesis is proposed for five visual fear signaling pathways in humans, based on an analysis of anatomical connectivity from primate studies and human functional connectvity and tractography from brain imaging studies. Earlier work has identified possible subcortical and cortical fear pathways known as the “low road” and “high road,” which arrive at the amygdala independently. In addition to a subcortical pathway, we propose four cortical signaling pathways in humans along the visual ventral stream. All four of these traverse through the LGN to the visual cortex (VC) and branching off at the inferior temporal area, with one projection directly to the amygdala; another traversing the orbitofrontal cortex; and two others passing through the parietal and then prefrontal cortex, one excitatory pathway via the ventral-medial area and one regulatory pathway via the ventral-lateral area. These pathways have progressively longer propagation latencies and may have progressively evolved with brain development to take advantage of higher-level processing. Using the anatomical path lengths and latency estimates for each of these five pathways, predictions are made for the relative processing times at selective ROIs and arrival at the amygdala, based on the presentation of a fear-relevant visual stimulus. Partial verification of the temporal dynamics of this hypothesis might be accomplished using experimental MEG analysis. Possible experimental protocols are suggested.
Frontiers in Systems Neuroscience 08/2015; 9(101). DOI:10.3389/fnsys.2015.00101
"). Because both early nonconscious and later conscious responses take place within the time window of a single volume acquisition in fMRI studies, the different functional values of neural activity in the same structure may be integrated or overridden (Brosch & Wieser, 2011; Costa et al., 2014; Luo et al., 2010). This limitation of fMRI has been partially circumvented by the use of methods with a better temporal resolution, such as electroencephalography (EEG) or magnetoencephalography (MEG), which have a temporal resolution on the order of milliseconds , but, on the other hand, have a poor spatial resolution and have been questioned when used to detect neural activity in subcortical structures (Andino, Menendez, Khateb, Landis, & Pegna, 2009; Cecere et al., 2014; de Gelder et al., 2002; Rossion et al., 2000). "
"Processing speed in the amygdala in response to sensory input is still under debate. Whereas some electrophysiological studies in humans have found amygdala peak activity as early as 40 ms after stimulus onset (Luo et al., 2010) and very fast face processing (Meeren et al., 2013), other studies report long latency responses in the amygdala with a time course more similar to activation via a cortical pathway (Gothard et al., 2007; Luo et al., 2010; Mormann et al., 2008). However, without an explicit network model or causal manipulation it is difficult to know whether the observed amygdala peak activity results from forward or recurrent activity. "
[Show abstract][Hide abstract] ABSTRACT: Human faces may signal relevant information and are therefore analysed rapidly and effectively by the brain. However, the precise mechanisms and pathways involved in rapid face processing are unclear. One view posits a role for a subcortical connection between early visual sensory regions and the amygdala, while an alternative account emphasises cortical mediation. To adjudicate between these functional architectures, we recorded magnetoencephalographic (MEG) evoked fields in human subjects to presentation of faces with varying emotional valence. Early brain activity was better explained by dynamic causal models containing a direct subcortical connection to the amygdala irrespective of emotional modulation. At longer latencies, models without a subcortical connection had comparable evidence. Hence, our results support the hypothesis that a subcortical pathway to the amygdala plays a role in rapid sensory processing of faces, in particular during early stimulus processing. This finding contributes to an understanding of the amygdala as a behavioural relevance detector.
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